This application relates to spread-spectrum communication systems and in particular to spread-spectrum communication systems for use on noisy network media, such as RF channels or AC power lines.
Spread-spectrum communication is a method whereby information is communicated using a bandwidth that greatly exceeds that required by information theory. These methods provide signals over a wide bandwidth, and with proper signal processing the communication is immune to large amounts of noise within that bandwidth. In chirp spread-spectrum methods, a signal burst known as a chirp is transmitted. Each chirp has energy spread across a frequency range. The frequency spread may be achieved by frequency sweeping or by such techniques as direct sequence coding. Chirps may be sent asynchronously, or at synchronous intervals, including as concatenated chirps. Data modulation of the chirp stream can be accomplished by means such as phase reversal modulation of the chirps or reversal of the frequency sequence of the chirp. A transversal filter in the receiver is matched to the chirp(s) expected, enabling individual chirps to be detected even on noisy network media such as power lines.
In a communication network, several transmitters and receivers may communicate with each other over a network medium. In certain networks, contention resolution and/or collision detection strategies are implemented to resolve situations in which two or more transmitters simultaneously require use of the network medium.
In U.S. Pat. No. 5,090,024, entitled Spread-Spectrum Communications System for Networks, and copending application, U.S. Ser. No. 07/863,213, entitled Timing for Spread-Spectrum Communication Across Noisy Media, systems are proposed for applying spread-spectrum communication to carrier-sense, multiple-access networks. Various embodiments are disclosed, including ones in which frequency-swept chirps are transmitted using ASK modulation for contention resolution and PRK modulation for data transmission.
Spread spectrum communication has been used for wireless, RF communication in local area networks. But these prior art RF systems have used pseudorandom direct sequence (typically maximal linear sequence) techniques, in which continuous synchronization of transmitter and receiver is required and maintained. These systems have generally required careful phase tracking mechanisms, and have been intolerant of any frequency offsets between transmitter and receiver carrier signals.